Would it be better to use approve?

[Joey]

In the arb.sol contract,
IERC20 erc20 = IERC20(tokenIn); erc20.transferFrom(msg.sender, address(this), amountIn);

It might be better to use erc20.approve()

[ccyanxyz]

Yes you're right, there are a lot improvements to make, so don't use this code in production.